1. Field of the Invention
The present invention relates to a tracking control system for an optical recording and reproduction apparatus which can record optical signals on and reproduce recorded signals from a recording medium by means of a light source such as a laser.
2. Description of the Related Art
In recent years, in order to improve optical recording and reproduction apparatuses (optical disc apparatuses), and achieve a reduction in size of the optical disc apparatus as well as high speed track seeking, separated optical systems using an optical pick-up have been used to achieve a reduction in the size of the movable part of the apparatus.
Before the introduction of separated optical systems, the major part of the optical system including the light source, the light detection unit and the objective lens etc. would all be installed as the movable part. For separated optical systems, this kind of optical system is arranged so as to separated into a fixed part and a movable part. For such a separated optical system, the utilization of a galvano-mirror and a linear motor (hereafter referred to as the "LM") as a tracking actuator for controlling the tracking of a light spot so that it follows a track on an optical disc (hereafter simply referred to as the "disc"), is already a known technique. Such a galvano-mirror is installed in the fixed part and is used as a fine actuator which deflects the light spot to maintain a precise position in the tracking direction. Such an LM is used as a crude actuator which moves the light spot in the radial direction over a wide area of the disc by moving the movable part, including the objective lens and suchlike, in the radial direction of the disc.
FIG. 1 is a block diagram showing the construction of the part of a conventional tracking control system which relates to a still jump operation. In FIG. 1, a side view of the optical system between the light source 1 and the photodetector 12, including the galvano-mirror 27 and the LM 28, is shown. To the left of the side view of the photodetector 12 is a figure showing the photodetection surface from the angle at which the light beam is incident on the surface.
As shown in FIG. 1, a tracking control system constructed according to the related art is comprised of a light source 1, a collimating lens 2, a beam splitter 3, a mirror 4, an objective lens 5, a spindle motor 6, a disc 7, a light beam 8, a convex lens 9, a cylindrical polarized beam splitter 10 (hereafter, the cylindrical PBS), a photodetector 12, a differential amplifier 13, another differential amplifier 14, a summing amplifier 16, a variable amplifier 17, a divider 18, a two state circuit 19, a phase compensation circuit 20, an equivalency filter 21, a phase compensation circuit 22, a sample and hold circuit 23 (hereafter, the S/H circuit), a summing amplifier 24, a galvano-mirror driving circuit 25 (hereafter, the GM driving circuit), an LM driving circuit 26, a galvano-mirror 27, an LM 28, a D/A convertor 31, a signal processing circuit 32, an A/D convertor 33 and a CPU 34.
The light source 1 consists of a semiconductor laser or suchlike and emits a coherent light beam 8.
The collimating lens 2 collimates the dispersed light beam 8 emitted by the semiconductor laser into a beam of parallel rays.
The beam splitter 3 allows the light beam 8 from the light source 1 to pass through it and reflects the light beam 8 reflected back by the disc 7 so as to be incident on the photodetector 12.
The mirror 4 reflects the light beam 8 from the light source 1 so as to pass through the objective lens 5.
The objective lens 5 focuses the light beam 8 from the light source 1 reflected by the mirror 4 on the disc 7.
The spindle motor 6 rotates the disc 7 at an approximate speed of 3600 rpm. By means of the spindle motor 6 rotating the disc 7, the light spot is made to move in a circumferential direction over the disc 7.
The disc 7 is a storage medium constructed so that there is a spiral track on its surface which includes a number of pits, and stores digital data according to the existence or not of such pits. The existence or not of such pits is detected according to the amount of light in the light beam 8 reflected off the surface of the disc 7.
The convex lens 9 focuses the light beam 8 reflected back off the disc 7 on the photodetection surface of the photodetector 12.
The cylindrical PBS 10 splits the light beam 8 reflected back off the disc 7 into the light beam 11 and the light beam 15. The light beam 11 is used for tracking control which is described below, while the light beam 15 is used for focusing control and suchlike which is also described below.
The photodetector 12 converts the amount of light in the light beam 11 and the light beam 15 incident upon its photodetection surface into electrical signals. As can be seen from the drawing of photodetection surface in the optical system drawn to the left of the photodetector 12 in FIG. 1, the photodetection surface of the photodetector 12 is divided into 4 areas, with the focus control signal being generated from the light beam 15 in the top right area of the photodetection surface, and the tracking control signal being generated from the light beam 11 in the bottom left area of the photodetection surface. The area of the photodetection surface used to generate the tracking control signal is further divided into 2 areas, with the 2 detected amounts of light being converted into 2 electrical signals, output A and output B. The area of the photodetection surface used to generate the focus control signal is further divided into 4 areas, with the amount of light in pairs of diagonally opposite corners of the photodetection surface being converted into electrical signals.
The differential amplifier 13 finds any difference in the electrical signals generated according to the amount of light detected by the focus control area of the photodetector 12 and outputs it as a focus error signal.
The differential amplifier 14 finds any difference in the electrical signals, output A and output B, generated according to the amount of light detected by the tracking control area of the photodetector 12 and outputs it as a tracking error signal. The summing amplifier 16 sums the amount of light in output A and output B and outputs it as the total light amount signal.
The variable amplifier 17 is an amplifier with an adjustable gain, and has its gain adjusted so that the amplitude of the tracking error signal is approximately constant at the output point "a" of the variable amplifier 17.
The divider 18 divides the tracking error signal which is the output of the variable amplifier 17 by the total light amount signal which is the output of the summing amplifier 16. The light beam incident on the photodetector 12 is subject to changes in the amount of light when the optical disc apparatus is recording or erasing data on the disc 7, or changes in the reflectivity of the disc 7, and corresponding to such changes in the amount of light in the light beam, the amplitude of the total light amount signal is modified in the same way as the tracking error signal. Therefore, by dividing the tracking error signal by the total light amount signal, the divider 18 can keep roughly constant the amplitude of the tracking error signal which it outputs, even when there are changes in the amount of light incident on the photodetector 12.
The two state circuit 19 converts the tracking error signal inputted from the divider 18 into a two state signal.
The phase compensation circuit 20 compensates the phase of the tracking error signal inputted from the divider 18, and by ensuring that there is a phase margin at the gain crossover frequency of the tracking control system which controls the driving of the galvano-mirror 27 (or the tracking servo loop) prevents oscillation in the tracking control.
The equivalency filter 21 is a low-pass filter with roughly equal transfer function characteristics to the tracking fine actuator which rotates the galvano-mirror 27.
The phase compensation circuit 22 compensates the phase of the tracking error signal which has passed through the equivalency filter 21, and, by ensuring there is a phase margin at the gain crossover frequency of the driving control system which controls the driving of the LM 28 (or the traverse servo loop), prevents oscillation in the tracking control.
The S/H circuit 23 simply outputs the LM driving value from the phase compensation circuit 22 to the LM driving circuit 26 when in sample mode, while, when in hold mode, it stores the LM driving value from the phase compensation circuit 22 at the point at which hold mode is initiated and outputs this held value.
The summing amplifier 24 adds together the tracking error signal which is the output of the phase compensation circuit 20 and galvano-mirror driving value which is the output from the D/A convertor 31.
The GM driving circuit 25 generates the GM driving voltage based on the galvano-mirror driving value outputted from the summing amplifier 24, and applies the generated GM driving voltage to the galvano-mirror 27.
The LM driving circuit 26 generates the LM driving voltage based on the LM driving value outputted from the phase compensation circuit 22 via the S/H circuit 23, and applies the generated LM driving voltage across the LM 28.
The galvano-mirror 27 includes a mirror part and a mirror moving part, so that the mirror moving part rotates the mirror in accordance with the GM driving voltage applied by the GM driving circuit 25. The mirror part reflects the light beam from the light source 1 which has passed through the beam splitter 3 towards the mirror 4 and reflects the light beam reflected back from the disc 7 towards the beam splitter 3.
The LM 28 is installed in the moving part which also contains the mirror 4 and the objective lens 5 and is connected by a flexible substrate or suchlike (hereafter, the flexi-substrate) to the LM driving circuit 26. The LM 28 is driven by the driving voltage applied by the LM driving circuit 26, so that it moves the movable part so as to follow a guide-rail (not shown in the drawings) which is positioned above the disc 7 in a radial direction to it.
The D/A convertor 31 converts the galvano-mirror driving value from the CPU 34 into an analogue signal.
The signal processing circuit 32 adjusts the amplitude of the total light amount signal from the summing amplifier 16 so as to conform to amplitude of the input voltage of the A/D convertor 33.
The A/D convertor 33 digitizes the total light amount signal from the signal processing circuit 32.
The CPU 34 detects any tracking errors of the light spot from the tracking error signal inputted from the two state circuit 19. The CPU 34 also reads the information such as the address of the light spot on the track from the total light amount signal inputted from the A/D convertor 33, as well as generating the driving value for the galvano-mirror 27 and the S/H signal which switches the S/H circuit 23 to hold mode/sample mode, and, by controlling the above construction elements, has the tracking operation performed.
The following is an explanation of an example of a still jump operation executed by a tracking control system constructed under the related art.
As shown in FIG. 1, once the light beam 8 emitted by the light source 1 has been collimated by the collimating lens 2, then it passes through the beam splitter 3, and is reflected by the galvano-mirror 27 which serves as the tracking fine actuator. The light beam 8 reflected off the galvano-mirror 27 is then reflected by the mirror 4 in the movable part so that it is then focused by the objective lens 5 on the surface of the disc 7 which is rotated by the spindle motor 6. The light beam 8 reflected back off the disc 7 then passes back through the objective lens 5 and is reflected by the mirror 4, before being reflected by the galvano-mirror 27. The light beam 8 reflected by the galvano-mirror 27 is then reflected by the beam splitter 3 so as to pass through the convex lens 9. The light beam 8 which passes through the convex lens 9 is then split by the cylindrical PBS 10 into the light beam 11 and the light beam 15.
The divided light beam 11 is then inputted into the photodetector 12. Output A and output B from the photodetector 12 are then inputted into each of the input terminals of the differential amplifier 14. The differential amplifier 14 then outputs the tracking error signal by calculating the difference between the output A and output B. The tracking error signal from the differential amplifier 14 is then inputted into the variable amplifier 17. The variable amplifier 17 then adjusts the amplitude of the tracking error signal so that it is roughly constant. The output of the variable amplifier 17 is then inputted into one of the input terminals of the divider 18. The total light amount signal from the summing amplifier 16 is inputted into other input terminal of the divider 18. The divider 18 then divides the output signal from the variable amplifier 17 by the output signal from the summing amplifier 16, in doing so restraining any change in amplitude in the tracking error signal generated as a result of changes in the light amount in the light beam 11.
The detection of a tracking error for the light spot based on a tracking error signal obtained by the process described above is known as the "push-pull method" (disclosed, for example, in Japanese Laid-Open Patent Publication No.49-60702). Additionally, if the total light amount signal which is the output of the summing amplifier 16 is demodulated by the CPU 34 after it has been processed by the signal processing circuit 32 and digitized by the A/D convertor 33, then while it is in the state wherein tracking control is being executed, the CPU 34 can read the address or the data which is stored on the disc 7 from the demodulated total light amount signal.